There are known in the prior art various dryers for drying the web formed in the course of making paper on a paper-making machine. One of the most widely used and effective dryers in the paper-making industry is the Yankee dryer. In this dryer, the web to be dried is passed over a dryer cylinder, the major portion of the periphery of which is enclosed by a hood. Hot drying air is supplied to the hood and is directed against the surface of the web and is then removed from the hood. Most of the systems are recirculating systems in which a certain portion of the air removed from the hood is returned thereto together with a predetermined amount of fresh, dry air. Another portion of the air removed from the hood is exhausted to the atmosphere.
Attempts have been made to optimize the drying rates without deleteriously affecting the quality of the paper produced. In order to achieve this result, relatively high temperature air and high velocity of the drying air has been employed. The excellent support for the web provided by the Yankee shell has made use of such high air velocities and mass flow possible.
One suggestion which has been advanced to increase the thermal efficiency of the Yankee dryer system is the preheating of the fresh air supplied to the system. Owing to the relatively small amount of fresh air used by a well designed Yankee unit, it has been discovered that little is gained by preheating the fresh air. Stated otherwise, it is a well established fact that fresh air usage and corresponding exhaust quantities are the major factors in determining the thermal efficiency of a given drying system. Specifically, in Yankee dryers the thrust of recent developments has been toward the reduction of fresh air input. Once the practical limit to the quantity of fresh air which must be used has been reached, the next step is the reduction of heat input in the recirculating system by preheating the fresh air. As has been pointed out, once the amount of fresh air required for the system has been minimized, preheating does not appear to have any appreciable effect in increasing the thermal efficiency of this system.
As the fresh air is reduced for a given water load input, the absolute humidity of the recirculating air increases and, through a given range, the system burner load shows a minimum point. The curve of burner load versus moisture pick-up at the minimum point is fairly flat. It can be demonstrated that once the system enters an absolute humidity range of 0.3 lb. of water per lb. of dry air up to 0.5 lb. of water per lb. of dry air, the system burner loads will be at their minimum.
From the foregoing, it will readily be apparent that optimization of the thermal efficiency of a dryer system can be achieved by controlling the operating point on the curve of burner load versus moisture pick-up. Such a control has not heretofore been achieved in modern dryer systems since no effective measure of the moisture pick-up in the air could be achieved. In dryers presently in use, the temperature of the air being removed from the dryer load ranges from about 500.degree. F. to 600.degree. F. Probes which have been used to measure humidity in relatively lower temperature installations have not proved effective, since their temperature tolerance is below about 200.degree. F. A further complication is the presence of fibers in the air which might contaminate any probe placed in a duct adapted to carry moisture laden air.